1. Field of the Invention
The present invention relates to an image processing apparatus, and more particularly to an apparatus for enlarging and reducing an image.
2. Description of the Prior Art
Various methods for enlarging and reducing the image have been proposed. For example, by doubling each bit data of a 4.times.4 matrix shown in FIG. 1 in both vertical (Y) direction and horizontal (X) direction, an enlarged image as shown in FIG. 2 is produced. For reduction, bits at predetermined positions [(X, Y) =(A, 0), (C, 0), (A, 2), (C, 2)] of the 4.times.4 matrix shown in FIG. 1 are sampled to produce a reduced image as shown in FIG. 3.
When the above enlarging and reducing method is applied to a conventional image (for example, character image generated by a pattern generator), no serious problem occurs, but when it is applied to a ditherred image, an image quality of a reproduced image may be degraded.
For example, when an image of a given density is ditherred by a dither matrix A shown in FIG. 4 (in which numerals indicate thresholds) and enlarged by a factor of two in both vertical and horizontal directions by the enlarging method described above, the enlarged image is same as a pattern obtained by ditherring by a dither matrix shown in FIG. 5. Thus, the reproducibility by the dither matrix A is lost. Since the ditherred image has a periodicity due to the dither matrix, a large change may occur in the reproduced image depending on the sampling bit positions.